This invention relates to a thermionic cathode of an electron gun used in many instruments such as an electron beam lithography system and/or a scanning electron microscope and more particularly to an improvement of a supporting device which supports the thermionic cathode tip in a thermally and mechanically stable manner.
Conventionally, the thermionic cathode of a tungsten hairpin type has been used. In electron beam lithography and/or scanning electron microscoper, a cathode tip consisting of a crystallization of lanthanum hexaborides having the caB.sub.6 type crystal structure supported by a pair of high anisotropic carbonaceous material in sandwiched manner at both sides is used.
A pair of heaters for supporting a cathode tip in a cathode device imparts heat of high degree to the thermionic cathode tip by a joule heat generated during energization to emit thermal electrons. The pair of heat generating support elements must not be thermally deformable and must keep their stable supporting ability and bring emitting ability of the thermionic cathode tip in such as lanthanum hexaboride, having the caB.sub.6 type crystal structure.
FIG. 1 shows a sectional view of a conventional device for supporting a thermionic cathode such as disclosed in U.S. Pat. No. 4,068,145. In the thermionic cathode supporting device, an emitter tip 1 is held by a pair of heaters 2a and 2b which, in turn, are held by a pair of electroconductive members 3 and 4 formed in a fork shape. The arrangement is such that each of the tip supporting heaters 2a and 2b is pressed in a direction to oppose each other by spring members 9 and 10. At the ends of the electroconductive members 3 and 4, there are formed divided portions in a further fork shape. At the end of the each inner divided portions 3a and 4a, the cathode tip 1 and the pair of tip supporting heaters 2a and 2b are simultaneously held. The spring members 9 and 10 are mounted on the further outer portion of the outer sections 3b and 4b of the divided portions of the electroconductive members so as to press the tip supporting heaters 2a and 2b in a direction to hold the cathode tip 1. The spring members 9 and 10 are made of such material as molybdenum which maintains resilence or elasticity even in a high temperature and formed in a line or a strip piece. The pair of conductive members 3 and 4 tend to be easily heated when the cathode tip 1 is heated and therefore, the spring members 9 and 10 are fixed to the external surface of the conductive members 3 and 4 by screws 11a and 11b so that the spring members are free from being heated by the heat from the cathode tip 1 and the tip supporting heaters 2a and 2b. A pair of pressure elements 12a and 12b which are made of insulating material such as porcelain and are pressed by free ends of the spring members 9 and 10, pass through the through holes 30a and 30b formed in the outer divided portions 3b and 4b and are slidably supported in axial direction by support pieces. The arrangement is such that each of the spring members 9 and 10 presses the pressure element 12a and/or 12b in an axial direction of the cathode to press inner divided portions 3a and 4a which, in turn, hold the tip supporting heaters 2a and 2b and the cathode tip 1.
In the above described prior art thermionic cathode supporting device referring to FIG. 1 of the present application, the pair of spring members 9 and 10 are directly fastened to the conductive members 3 and 4 by screw members, and consequently, the heat in the conductive members 3 and 4 raises the temperature of the spring members 9 and 10. Thus, the spring members 9 and 10 lose their elasticity by the long time heating although their materials are not so influenced by heat. The loss of elasticity results in the variation of contact resistance between the tip supporting heaters and the cathode tip. The variation of the contact resistance, undesirably varies the temperature of the cathode tip 1. It is to be noted that the outer sections 3b and 4b of the divided portion of the conductive members 3 and 4 serve only as members for mounting the spring members 9 and 10 and as a slide guide thereof, but can not serve to prevent the heat radiation from the top portion of the cathode to the spring members 9 and 10. It is further to be noted that the spring members are formed in a manner to extend outwardly so that the cathode device becomes undesirably large in size. Normally, the electroconductive materials are heat conductive materials. Therefore, as the conductive members become larger in size, more heat is conducted, which results in more heat loss and requires more electric power. As disclosed hereinabove, holding the force of each of the tip supporting heaters 2a and 2b to support the emitter tip 1 is imparted by the spring members 9 and 10. However, in the arrangement, micro adjustment of the pressing force is not practicable.